A pulsed current source such as the Analog Modules Model 771 or 778 laser diode drivers may be used to drive a single or stack of laser or light emitting diodes. However, this can be inefficient and dissipative because a storage capacitor bank needs to be charged to a voltage substantially higher than the laser diode voltage requirement. Nevertheless, drawing the laser current from a storage capacitor bank rather than from a power supply and filter may be beneficial because a power supply would have to be rated at a large power to supply the diode current. There is also a difficulty in maintaining regulation in a power supply over the range of pulse current draw required, zero to maximum diode current.
The Model 771 incorporated a variac to manually set this voltage. A power supply charges up the storage capacitor, or multiple capacitors (bank), to a regulated voltage level between laser pulses and a current control device such as an FET (field effect transistor) is used to regulate the current, in conjunction with a current sensing device. The factors that require this higher regulated bank voltage may include the following (any of which may be referred to as “losses”):                droop in the storage capacitor voltage during the pulsed discharge;        ESR (equivalent series resistance) losses in the storage capacitor during the discharge;        ohmic loss across the current control device, FET, or current control transistor;        loss in the current sensor which measures the current flow;        capacitor value initial tolerance and end of life allowance;        temperature effects on capacitor value, ESR, and other component parameters; and        variations in pulse width required (without the current control device saturating).        
When supplying power from the storage capacitor(s) to the load (such as laser or light emitting diodes), particularly in a pulsed-mode operation, the voltage in the storage capacitor(s) will droop during the current pulse but must always be adequate to power the current control device without saturation otherwise the required current will fall at the end of the pulse.
For the above reasons, the storage capacitor is typically charged 5 to 10 volts above the laser diode required voltage with a significant dissipation occurring in the current control device determined by the instantaneous excess voltage times the current flow. This dissipation causes this design to be inefficient and limited by the safe forward operating conditions of the control device. Each laser diode has a voltage of about 2.5 volts, but frequently multiple diodes are connected in series to generate more laser power. This configuration requires a higher drive voltage for the correct current.
U.S. Pat. No. 7,262,584 discloses efficient fast pulsed laser or light-emitting diode driver. A capacitor connected to the output of a multiphase power converter, and a current-driven device (e.g., LED or laser diode) is also connected to the power converter output. A solid state switch (FET or IGBT) is connected in series with the current-driven device. Means are provided for sensing current through the current-driven device. An error amplifier compares sensed current through the current-driven device with a current level demand signal and controls the output of the power converter. Means are provided for turning the switch on and off and may be (i) a fast comparator receiving a voltage reference signal at one input and the current level demand signal at another input, and outputting the switch on/off signal to the switch or (ii) an externally-generated logic signal provided directly to the switch.
U.S. Pat. No. 7,756,173 discloses laser diode driver with adaptive compliance voltage. A laser diode driver with means for adjusting the compliance voltage to allow a current source to accurately reproduce a current command while simultaneously minimizing the power dissipation of the current source. For a slowly-varying or DC current command, the compliance voltage is continuously adjusted to limit the power dissipation of the current source to below a predetermined minimum. For a pulsed current waveform, the compliance voltage is maximized during periods of zero or low current demand so that sufficient energy is stored to faithfully reproduce the leading edge of a pulsed current command, and reduced during the plateau portion of a pulsed current command to minimize the power dissipation of the current source.